1. Field of the Invention
The present invention relates to a system and method for modifying a live cornea by ablating an exterior or interior surface of the live cornea with laser radiation, and then eroding the irradiated surface with a mechanical eroding tool to remove a coagulated portion of the cornea created by the laser ablation. More specifically, the present invention relates to a system and method which uses an infrared, and preferably an erbium:YAG, laser to ablate a portion of an exterior or interior surface of a live cornea, and then a mechanical eroding tool which includes a material that is placed in contact with the irradiated surface of the cornea to physically remove (e.g., erode) a coagulated portion of the cornea created by the laser ablation, to alleviate cloudiness in the modified live cornea and reduce the healing time.
2. Description of the Related Art
Various surgical techniques presently exist for correcting ametropic conditions of the eye, such as myopia, hypermetropia or hyperopia, and astigmatism. In a normal emetropic eye, which includes a cornea, lens and retina, the cornea and lens cooperatively focus light entering the eye from a far point (i.e., infinity) onto the retina. However, in an ametropic eye, the cornea and lens are incapable of correctly focusing the far point on the retina.
For instance, in a myopic eye, the cornea or lens has a refractive power stronger than that of the cornea and lens of an emetropic eye, or the axial length of the myopic eye is longer than that of a normal emetropic eye. The stronger refractive power or longer axial length causes the far point to be projected in front of the retina.
Conversely, a hypermetropic or hyperopic eye has an axial length shorter than that of a normal emetropic eye, or a lens or cornea having a refractive power less than that of a lens and cornea of an emetropic eye. This lesser refractive power or shorter axial length causes the far point to be focused in back of the retina.
An eye suffering from astigmatism, on the other hand, has a defect in the lens or shape of the cornea. Therefore, an astigmatic eye is incapable of sharply focusing images on the retina.
Perhaps the most common technique for correcting the vision in an eye suffering from ametropic conditions is the use of glasses. In this technique, a lens is placed in front of the eye (i.e., in the form of glasses or a contact lens) to compensate for the focusing defect in the cornea and lens of the eye. However, glasses and contact lenses are often lost and have to be replaced.
Surgical techniques have also been developed to correct these more severe forms of ametropia. Many of these surgical techniques involve the modification or reshaping of the surface of the cornea, which changes the refractive power of the cornea and thus corrects the focusing defect in the eye. The shape of the cornea can be modified by surgically cutting the cornea with a microkeratome, for example, or by inserting an organic or synthetic artificial lens inside the cornea.
A further surgical technique employs the use of ultraviolet and shorter wavelength lasers which are commonly known as excimer lasers that produce pulsed ultraviolet radiation. In one type of laser surgical technique, the pulsed ultraviolet radiation is directed onto the outer surface of the cornea to ablate portions of the cornea and thus modify or reshape the surface of the cornea. However, this technique (photorefractive keratectomy) is generally ineffective in correcting high myopia of 6 diopters or greater, and is also ineffective in correcting severe astigmatisms and severe forms of hypermetropia or hyperopia.
Another surgical technique known as laser in situ keratomycosis (LASIK) has been previously developed by the present inventor as disclosed in U.S. Pat. No. 4,840,175 to Peyman, the entire contents of which is incorporated herein by reference. In this technique, a portion of the front of the live cornea can be cut away in the form of a flap having a thickness of about 160 microns. This cut portion is removed from the live cornea to expose an inner surface of the cornea. A laser beam generated by an excimer laser is directed onto the exposed inner surface to ablate a desired amount of the surface up to about 150-180 microns deep. A cut portion is than reattached over the ablated portion of the cornea, and assumes the shape conforming to that of the ablated portion.
Although the above-mentioned laser surgery techniques are very effective in modifying the shape of the cornea and thus correcting the ametropic conditions discussed above, several problems with excimer lasers exist. For instance, an excimer laser is very expensive. Typical excimer lasers, such as the argon-fluoride, krypton-fluoride and xenon-chloride lasers, can cost between $400,000 and $500,000. Furthermore, these types of excimer lasers usually include halogen gases such as fluorine and chlorine, which are highly toxic and require special handling. Specifically, if the chlorine or fluorine gas needs to be replaced, a skilled technical often needs to perform the servicing.
In an attempt to alleviate these problems associated with excimer lasers, the present inventor has experimented with infrared lasers, such as an erbium:YAG (Er:YAG) laser (and also Nd-YAG, HF and CO lasers), to perform the above-mentioned laser surgery techniques. The infrared lasers emit light having a wavelength within the range of 0.8 micrometers (i.e., microns) to 5.5 microns which is essentially within the infrared light spectrum. Some infrared (e.g., Er:YAG) lasers are portable, solid state devices which typically cost between $50,000 and $100,000, which is much less expensive than excimer lasers. Furthermore, the infrared (e.g., Er:YAG) lasers typically are more durable and thus have a useful life span longer than that of the excimer type lasers. Also, the Er:YAG lasers use no toxic gas compared with the excimer lasers, and thus are much safer and easier to use.
As stated above, unlike excimer lasers which emit pulsed ultraviolet light (e.g., light having a wavelength of, for instance, 193 nm), the Er:YAG lasers emit light within the infrared range having a wavelength at or about 2.9 microns. Examples of laser surgery experiments performed with Er:YAG lasers are discussed in the following articles: Peyman et al. entitled "Long-Term Effect of Erbium-YAG Laser (2.9 .mu.m) on the Primate Cornea", published in International Ophthalmology, Vol. 15, pp. 249-258 (1991); Peyman et al. entitled "Corneal Ablation in Rabbits Using an Infrared (2.9 .mu.m) Erbium:YAG Laser", published in Ophthalmology, Vol. 96, No. 8, pp. 160-70 (August 1989); Seiler et al. entitled "Erbium: YAG Laser Photoablation of Human Cornea", published in American Journal of Ophthalmology, Vol. 120, No. 5, pp. 668-9 (November 1995), each of which is incorporated by reference herein in its entirety.
Although the infrared (e.g., Er:YAG) lasers are advantageous over the excimer lasers for the reasons discussed above, certain problems are experienced when infrared (e.g., Er:YAG) lasers are used to perform corneal modification. In particular, although the infrared radiation produced by an Er:YAG laser does ablate the surface of the live cornea to which it is directed, the ablation results in coagulation of the underlying corneal tissue to a depth of about 3-6 microns. This coagulation causes areas in the cornea to become soft and cloudy and thus, obstructs the vision of the eye. Although the coagulated tissue will eventually heal, the healing period is typically about three months, which is significantly longer than the healing period (e.g., about three days) for a cornea that was modified by similar techniques performed with an excimer laser.
Accordingly, a need exists for a system which can use an infrared and, in particular, an Er:YAG laser, to perform laser surgery on the eye to modify the cornea, but which does not experience the drawbacks associated with infrared laser (e.g., Er:YAG) eye surgery as known in the art.